Glow plugs, which are used in assisting a start of an internal combustion engine such as a diesel engine, include a tubular housing, a heater member which is energized to be heated, and the like. As the heater member, a ceramic heater having a heating element made of a conductive ceramic and a sheathed heater having a heating coil are adopted as from time to time.
In addition, the housing includes a screw portion for mounting to the internal combustion engine and a pressure contact portion which is brought into pressure contact with a seat surface provided to the internal combustion engine when the screw portion is screwed into a mounting hole in the internal combustion engine to ensure gastightness in a combustion chamber. In general, the pressure contact portion is configured by a front end surface of the housing which is formed so that an outer diameter thereof gradually reduces as it extends towards a front end in the direction of an axis thereof (for example, refer to Patent Literature 1 or the like). Additionally, when the glow plug mounted to the internal combustion engine, a compression force (an axial force) along the direction of the axis will be applied to a portion (a front-end-side body portion) of the housing which is located between the pressure contact portion and the screw portion. Then, the pressure contact portion is brought into pressure contact with the seat surface by a contact pressure which corresponds to the axial force.